Thread: 55 Wagon Progress

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Nice work, but I've got to ask why you go with no gap on the 2 pieces??? The general rule for years from most of the fabricators I know, a bunch of the old tin benders, and the folks who make the welders and wire we use has been to leave a .030 to .040 gap to allow the weld to penetrate properly..... Not criticizing your work, just wondering????

Using no gap is largely for consistency. Any weld causes shrinking, and in the case of a low crown panel, there is typically insufficient shape in the panel to resist any of this movement. So in a case like welding horizontally through the center of a quarter panel (adding a repair panel at the bottom) any shrinking tends to pull from the adjacent panels, losing some of the crown in the process. It's here especially why the "Leave a gap" theory causes so much headache. As the heat causes the shrink, if someone did leave a gap, there is nothing to keep the panels from moving even that much more closer to one another. The first tack weld will pull the panels closer together, the next even more. Now when you planish to stretch the weld and HAZ, you'll also have that panel movement (and inconsistent movement) to try and compensate for, making the task more work. So if you did leave a gap, that becomes a larger hurdle in trying to restore the panel's crown. An absolutely tight butt joint between the two panels (or to the best of one's ability) is crucial to keep the panels from pulling together, losing even more crown/panel shape. This consistency even starts with fitting up the panel. Tight joints in some areas and wider gaps in others will result in more welding and thus heat in those wider gaps, for more shrinking, more planishing required. As for using the gap to allow for weld penetration, IMO the welder should be properly set up with adequate heat for full weld penetration (without a gap), and with sufficient wire feed speed to not blow holes. This should be done first on scraps before welding on your car/project. Consistency throughout the process, from panel fitment to welding methods to planishing, will make for better consistency in the results.